1. Field of the Invention
The invention relates generally to the field of communications. More particularly, the invention relates to digital subscriber loop (DSL) communications. Specifically, a preferred implementation of the invention relates to extending the range of an asymmetric digital subscriber loop (ADSL). The invention thus relates to ADSL of the type that can be termed extended.
2. Discussion of the Related Art
Conventional telephony, often called plain old telephone service (POTS), is provided to customers over copper cable. This copper cable can be termed a subscriber loop or a subscriber line. Modern loop plant designs specify the use of 26-gauge cable for short to medium loop lengths with 24-gauge cable used to extend the range. Legacy loop plant includes cable of 22-gauge as well as 19-gauge.
At the customer premises, a telephone set is typically connected to the cable. The other end of the cable is connected to a line circuit module in the service provider""s central office (CO). Switches terminating customer loops at the central office are regarded as Class-5 switches and provide a dial-tone. The customer premise equipment (CPE) can include a personal computer (PC) modem.
Older central office switches were analog in nature and were unable to provide a broad range of services. Modern central office switches are digital. Digital switches include codecs in the line circuit to do the bilateral analog-digital (A/D) conversion; the transmission over the loop is analog and the signals occupy a frequency band of up to (approximately) 4 kHz. Conventional telephony codecs convert at an 8 kHz sampling rate and quantize to 8 bits per sample corresponding to a net bit rate of 64 kbps (or xe2x80x9cDS0xe2x80x9d).
With the advent of digital terminal equipment, such as personal computers, modems were developed to carry digital bit streams in an analog format over the cable pair. Because of the 4 kHz constraint imposed by the A/D converter in the line circuit, the data rate of such transmission is limited and is typically 9.6 kbps. More elaborate schemes have been proposed which permit higher bit rates (e.g. V.34 which can do in excess of 28.8 kbps). More recently, there are schemes that xe2x80x9cspoofxe2x80x9d the D/A converter in the line-circuit operate at bit rates as high as 56 kbps in the downstream direction (from CO to CPE). With increasing deployment of, and consequently demand for, digital services it is clear that this bit rate is insufficient.
An early proposal to increase the information carrying capacity of the subscriber loop was ISDN (xe2x80x9cIntegrated Services Digital Networkxe2x80x9d), specifically the BRI (xe2x80x9cBasic Rate Interfacexe2x80x9d) which specified a xe2x80x9c2B+Dxe2x80x9d approach where 2 bearer channels and one data channel (hence 2B+D) were transported between the CO and the CPE. Each B channel corresponded to 64 kbps and the D channel carried 16 kbps. With 16 kbps overhead, the loop would have to transport 160 kbps in a full duplex fashion. This was the first notion of a Digital Subscriber Loop (xe2x80x9cDSLxe2x80x9d) (or Digital Subscriber Line). However, this approach presumed that POTS and 2B+D would not coexist (simultaneously). The voice codec would be in the CPE equipment and the xe2x80x9cnetworkxe2x80x9d would be xe2x80x9call-digitalxe2x80x9d. Most equipment was designed with a xe2x80x9cfall-backxe2x80x9d whereby the POTS line-circuit would be in a xe2x80x9cstand-byxe2x80x9d mode and in the event of a problem such as a power failure in the CPE, the handset would be connected to the loop and the conventional line-circuit would take over. There are several ISDN DSLs operational today.(1-2) 
Asymmetric digital subscriber loop (ADSL) was proposed to provide a much higher data rate to the customer in a manner that coexisted with POTS. Recognizing that the spectral occupancy of POTS is limited to low frequencies, the higher frequencies could be used to carry data (the so-called Data over Voice approach). Nominally, ADSL proposed that 10 kHz and below would be allocated to POTS and the frequencies above 10 kHz for data. Whereas the nominal ADSL band is above 10 kHz, the latest version of the standard specifies that the xe2x80x9cuseablexe2x80x9d frequency range is above 20 kHz. This wide band between 4 kHz and the low edge of the ADSL band simplifies the design of the filters used to segregate the bands.
Furthermore, it was recognized that the downstream data rate requirement is usually much greater than the upstream data rate requirement. Several flavors (xe2x80x9cClassesxe2x80x9d) of ADSL have been standardized, involving different data rates in the two directions. The simplest is Class-4 which provides (North American Standard) 1.536 Mbps in the downstream direction and 160 kbps in the upstream direction. The most complicated, Class-1, provides about 7 Mbps downstream and 700 kbps upstream.(3-4) 
A stumbling block in specifying, or guaranteeing, a definite bit rate to a customer is the nature of the loop plant. Customers can be at varied geographical distances from the central office and thus the length of the subscriber loop is variable, ranging from short (hundreds of feet) to long (thousands of feet) to very long (tens of thousands of feet). The essentially lowpass frequency response of subscriber cable limits the usable bandwidth and hence the bit rate.
Moreover, loops longer than (approximately) 18 thousand feet have a lowpass characteristic that even affects the voiceband. Such loops are specially treated by the addition of load coils and are called xe2x80x9cloaded loopsxe2x80x9d. The principle is to splice in series-inductors which have the impact of xe2x80x9cboostingxe2x80x9d the frequency response at (approximately) 4 kHz with the secondary effect of increasing the attenuation beyond 4 kHz very substantially. In these loaded loops, the spectral region above 10 kHz is unusable for reliable transmission. Consequently, the categorical statement can be made that DSL (including ADSL, xe2x80x9c2B+Dxe2x80x9d, and other flavors of DSL) cannot be provided over long loops and definitely cannot be provided over loaded loops.
Heretofore, there has not been a completely satisfactory approach to providing DSL over long loops. Further, there has not been a satisfactory approach to providing DSL over loaded loops. What is needed is a solution that addresses one, or both, of these requirements. The invention is directed to meeting these requirements, among others.
There is a need for the following embodiments. Of course, the invention is not limited to these embodiments.
One embodiment of the invention is based on a method, comprising: utilizing a circuit having a first end and a second end, said circuit having a first amplification interface connecting said first end to said second end in a first direction, and a second amplification interface connecting said second end to said first end in a second direction; adapting said first amplification interface to provide a first gain adjustment as a function of a first attenuation of a first communication by a first direction impedance from said transmission medium while transmitting in said first direction, said first communication within a first frequency range over said transmission medium from said first end to said second end; and adapting said second amplification interface to provide a second gain adjustment as a function of a second attenuation of a second communication by a second direction impedance from said transmission medium while transmitting in said second direction, said second communication within a second frequency range over said transmission medium from said second end to said first end. Another embodiment of the invention is based on an apparatus, comprising: a modulator for transmitting a first communication, in a first direction over a transmission medium, said modulator operably coupled to a first amplification interface for providing a first gain adjustment, based on a first attenuation of said first communication in said first direction by a first direction impedance of said transmission medium; and a demodulator operably coupled to said modulator, for receiving a second communication, in a second direction over said transmission medium, said demodulator operably coupled to a second amplification interface for providing a second gain adjustment, based on a second attenuation of said second communication in said second direction by a second direction impedance of said transmission medium.
These, and other, embodiments of the invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating various embodiments of the invention and numerous specific details thereof, is given by way of illustration and not of limitation. Many substitutions, modifications, additions and/or rearrangements may be made within the scope of the invention without departing from the spirit thereof, and the invention includes all such substitutions, modifications, additions and/or rearrangements.